Drakkith said:
Can you elaborate on this? I wasn't aware that fusion plants would produce tritium.
The reaction people contemplate for commercial fusion is D-T. This is the reaction that people believe is the easiest to produce, and probably the easiest to keep going or make happen often enough to produce commercially useful power levels.
The biggest challenge with D-T fusion is that it produces a massively high energy neutron.
https://en.wikipedia.org/wiki/Fusion_power#Deuterium.2C_tritium
There are designs that want other reactions besides D-T, but they are harder to make work. D-T is hard enough.
So you get a 14 MeV neutron. And this is about 80% of the total energy of the fusion reaction. So you need a way to catch them and get their energy out.
Catching 14 MeV neutrons is a big challenge. At the same time, you want to get some more tritium to replace the T you burn. And the material you use to catch the neutrons has to be such that you can get the heat out and into the material. And you need to then be able to get that heat into some kind of electrical generator, most often some kind of steam generator driving a turbine. And the material has to be such that it can stand up to operating under conditions of this sort for at least some years. Otherwise it won't be commercial.
So the usual solution to these constraints that people come up with is a blanket of molten lead with lithium in it. The neutrons bounce around in the lead, giving up their energy. They also find lithium and produce tritium. Then the lead is circulated to heat a boiler.
You need a blanket of lead approximately 2 meters thick to adequately catch enough energy. Depending on what the structures around the reactor can deal with, you might need to increase that a little.
Now lead has a disagreeable aspect when you use it as shielding. It's great for gamma shielding. But for neutrons it has this feature where a high energy neutron will knock a neutron off a lead nucleus. It is sometimes referred to as "spalling." So it means you start with a 14MeV neutron. And suddenly you have two neutrons at, say, 4 MeV and 5 MeV, and the lead nucleus gets the rest. Or possibly it emits a gamma with the rest, and the gamma is quickly absorbed in the lead.
But now you begin to see the issue. One neutron at 14 MeV can become more than one neutron at lower energies. Sometimes 3 or 4. And sometimes those extra neutrons find lithium to convert to tritium.
A couple years ago I was on a contract to a fusion reactor research company. Their design does the stuff I described here. (That's me in front of their test device in my avatar.) I did some MCNP work to estimate things like the heat load to the structure at the outer surface of the lead. That load was disagreeable, but could probably be managed. But I also estimated the amount of tritium produced per tritium used up in the reaction. And it was much more than 1. Depending on the details of the core and the lead and how much lithium, it might be more than 2 tritiums produced per tritium burned.
There has been some work on being able to dial this number. I did some myself in my spare time. But it was not part of the contract, so I didn't spend that much time. And there are a lot of constraints on the lead. You have to get a very large percentage of the neutron flux absorbed in the lead. Neutrons do a disagreeable thing when they hit iron. Even very slow neutrons will get absorbed by the iron, and release a gamma. That means even very low energy neutrons will heat the iron parts of the reactor structure. So it's a problem. If you wind up with a 1000 MW fusion reactor, and the iron parts of the core get heated to by 1 percent of that, that's 10MW into the iron parts of the core. This is a challenge.
As well, most fusion reactor research is much more concerned with getting the plasma at all, never mind how to get the heat out. So maybe this number can easily be dialed to what is required to keep the reaction fed. But as of this time, it has not been.
